The invention concerns a coating system for the coating of traveling webs of material, notably of paper or cardboard. Meant by a coating system is a kind of system featuring rolls of which at least one is a web support roll while another is a metering roll and effects a coating or impregnation of a traveling paper web with a coating mixture or other coating or impregnating substance.
EP 0 362 174 A describes a system in which the metering roll has a cylindrical core and, wound on it in the form of a threading, a wire with a small diameter, which is stated, e.g., to be 0.3 mm. According to the references in this document, the cylindrical core diameter may be about 300 mm. Small rolls of reduced diameter and of such structure have so far been used frequently and called grooved roll doctors. The diameter of these so-called roll doctors ranged generally between 10 and 20 mm. The disadvantage of all of these systems is the wear of the fine wire lashing of the substrate core, both with the grooved roll doctor and the metering roll provided with a wire lashing. Such wear occurs particularly in the case of pigmented coatings. As a result, the necessary constant rate of application flow is no longer given over the service life of the grooved metering roll or roll doctor. Namely, as the bulges of the wires or protuberances forming the grooves wear down, a reduction occurs of the passage cross section between metering roll and web support roll, which entails a steady reduction of the coating weight.
Another great disadvantage, however, is the occurrence of a considerable jump in coating weight as a worn metering system is replaced by a new one. This leads to quality fluctuations in the finished product, which in part can no longer be tolerated. Therefore, the problem underlying the invention is to reduce the wear of the metering roll or doctor.